Prevent bad metadata_size in extended handshakes from stalling completion of metadata
Occasionally bad peers send ridiculous or incorrect metadata_size in their handshakes. If the first acceptable size is wrong, and too small, we'll keep failing metadata completion. If it's too large, honest peers will never send us the pieces we're asking for and we'll never complete the metadata. Now we just adjust the expected metadata size, and keep retrying until we finish. Additionally, we can now request metadata after sending initial messages.